1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to zoom lenses of the rear focus type with a color separation prism in the space between the rear vertex and the CCD. Still more particularly, this invention relates to high range, large relative aperture zoom lenses of the rear focus type which provide, despite securing of such a long back focal distance, increase of the zoom ratio while still permitting the diameter of the front lens members to be minimized.
2. Description of Related Art
Recently, home video cameras of ever smaller size and lighter weight have been developed. Along with this development, a remarkable advance is seen even in decreasing the bulk and size of its taking zoom lens. In particular, efforts are being devoted to shortening of the total length of the entire lens system, reduction of the diameter of the front lens members and simplification of the form and the construction and arrangement of the constituent parts.
To attain these ends, one means is to move a lens unit other than the front or first lens unit to effect focusing. Such a zoom lens is known as the so-called "rear focus type".
In general, the rear focus type of zoom lens has many advantages over the type which performs focusing by moving the front lens unit. For example, it becomes easier to improve the compact form of the entire lens system. Close-up photography, particularly supershort focusing, becomes possible. Further, since the focusing lens unit is of small size and light weight, because the required driving torque for moving the focusing lens unit is reduced, rapid focus adjustment can be carried out.
Such a rear focus type of zoom lens is disclosed in, for example, Japanese Laid-Open Patent Applications Nos. Sho 62-206516, Sho 62-215225 and Sho 62-24213, in which the zoom lens comprises, in order from an object side, a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit, the second lens unit being axially moved to vary the focal length, and the fourth lens unit being axially moved to compensate for the image shift with zooming and also to effect focusing.
Also, in Japanese Laid-Open Patent Applications Nos. Hei 4-43311, Hei 4-153615, Hei 5-19165, Hei 5-27167 and Hei 5-60973, there are disclosed examples of the fourth lens unit consisting of one or two positive lenses. Also, in Japanese Laid-Open Patent Application No. Hei 5-60974, there is disclosed a zoom lens whose fourth lens unit is constructed with positive and negative lenses, totaling two lenses.
Further, in Japanese Laid-Open Patent Applications Nos. Sho 55-62419, Sho 62-24213, Sho 62-215225, Sho 56-114920, Hei 3-200113, Hei 4-242707, Hei 4-343313 and Hei 5-297275, there are disclosed, in their embodiments, zoom lenses in which the third and fourth lens units each are constructed with a positive lens and a negative lens, totaling two lenses.
Another recent trend in the art of video cameras has come along with enhancement (digitization) of the performance of video decks. To measure up to this, the image quality must ever more advance. One method of attaining the high image quality is to split the image by using a color separation optical system. Lenses that are suited to be used with this system have been proposed as disclosed in Japanese Laid-Open Patent Applications Nos. Hei 5-72474, Hei 6-51199, Hei 6-337353, Hei 6-347697, Hei 7-199069 and Hei 7-270684, etc.
As mentioned above, in general, for the zoom lenses, in view of achieving valuable reduction of the diameter of the front lens members with the size of the entire system at a minimum, the so-called "rear focus" configuration is more suitable than that of focusing by the first lens unit.
In the above-mentioned Japanese Laid-Open Patent Applications Nos. Hei 4-026811 and Hei 4-88309, however, their configurations are hardly amenable to dispose the color separation prism.
In the zoom lenses disclosed in the above-mentioned Japanese Laid-Open Patent Applications Nos. Hei 4-43311, Hei 4-153615, Hei 5-19165, Hei 5-27167 and Hei 5-60973, the zoom ratio is 6 to 8 or thereabout. For zoom lenses of higher ranges than this, the variation of chromatic aberrations with zooming would becomes too large to correct well. It is, therefore, difficult to assure maintenance of sufficient optical performance. Even the examples disclosed in the above-mentioned Japanese Laid-Open Patent Application No. Hei 5-60974, too, have a zoom ratio of 8, so that no sufficient increase of the range is achieved.
Further, in the examples disclosed in the above-mentioned Japanese Laid-Open Patent Applications Nos. Sho 55-62419, Sho 56-114920 and Hei 3-200113, either the first lens unit or the third lens unit, too, is made to move during zooming. This leads to increase the complexity of the operating mechanism. These zoom lenses are, therefore, not suited to achieve improvements of the compact form. In the examples disclosed in the above-mentioned Japanese Laid-Open Patent Applications Nos. Hei 4-242707, Hei 4-343313 and Hei 5-297275, construction and arrangement are made such that the third lens unit has a large air space. In addition, a negative lens included in the third lens unit is relatively weak in refractive power. In application to high range zoom lenses, therefore, this does not become a type that assures sufficient correction of the chromatic aberrations the third lens unit would produce. Furthermore, in the example disclosed in the above-mentioned Japanese Laid-Open Patent Application No. Hei 5-297275, a negative meniscus lens included in the third lens unit is made to have a strong concave curvature toward the image side. This is advantageous for obtaining the telephoto form, but does not favor the negative lens to treat the flare component of higher orders the positive lens produces. Such a type is, therefore, disadvantageous to be used in the large relative aperture, high range zoom lenses.
In the above-mentioned Japanese Laid-Open Patent Applications Nos. Hei 5-72474, Hei 6-51199, Hei 6-337353, Hei 6-347697, Hei 7-199069 and Hei 7-270684, any of their embodiments has as low a zoom ratio as 10 to 12. Thus, the sufficient increase of the range is also not achieved.